1. Field of the Invention
This invention relates generally to operational amplifier circuits and more particularly to an operational amplifier circuit that is selectively reconfigured as a filter, a sample and hold circuit, and a comparator.
2. Description of the Prior Art
Typically, operational amplifiers are used in a variety of circuits. One application of an operational amplifier is in a filter circuit whereby low frequency poles are effected by the use of external components. Operational amplifiers are also used to sample a signal onto a feedback capacitor and temporarily hold the signal. Such circuits are known as sample and hold circuits and are used to transfer the charge produced by the sampled signal to a capacitance storage device. Another typical application of an operational amplifier is in a comparator circuit in which the difference between two input signals is amplified. Comparators are utilized extensively in analog to digital and digital to analog conversion circuits.
Disadvantages in the prior art are evident when more than one of these operational amplifier circuits is desired in a single integrated circuit application. Typically, separate operational amplifiers are used for each circuit application. Apart from the large die areas that are required when multiple operational amplifiers are used, each operational amplifier adds its own offset error into the system, and offset compensation circuits increase the circuit's size. When two or more circuit functions have been combined, external components such as sample and hold capacitors and coupling capacitors have generally been required. As a result of the limited ability to use a single operational amplifier for multiple functions, the prior art tended to dedicate each operational amplifier to specific circuit functions.